The most important fact in this thread is that almost nobody commenting has actually done an ultra. I can lend a bit of experience here. Unfortunately I can't run a 2:15 marathon so I'm significantly slower than everybody here and my opinion weans very little.
What everybody is missing in this argument is the physiological differences in these events. Clearly elite road racers have the aerobic engine necessary to excel at most running events. What they may or may not have is the musculature in the legs to handle an extreme trail event. Running 27:00 10k on the track requires a huge aerobic engine and oxygen transport system. It also requires almost zero leg strength, particularly quad strength. This is evident if you look at any picture of elite track athletes. And don't show me a string bean with muscle definition because he has zero fat. That doesn't mean he has developed muscles, it means he has little fat.
Running for many hours up and down steep hills, and navigating rocks and roots, etc requires a completely different set of physical abilities. Until you've done one, you have zero idea what happens to your legs. The greatest engine in the world can't run fast when your legs are completely broken down and you're forced to walk backwards down the slightest downhill, or are unable to even jog up an incline because your legs will not respond.
The Kenyans would never be short of breath in an ultra...I think we can all agree here. But these guys are like fragile birds. On certain courses they would stand absolutely no chance, and in most ultras I'd be shocked if their legs held up long enough to win.
The ultra guys with the legs sturdy enough to handle the terrain and a solid VO2 are the ones winning races. Not a common combination. The good news for road racers if they wanted to dominate ultras is that leg strength for ultras can be developed. A huge aerobic engine is something you either have or you don't.